The subject matter described herein relates generally to methods for detecting vibration nodes, and more specifically, to methods for detecting vibration nodes between a non-collocated sensor and actuator pair in a rotor-dynamic machine having an active magnetic bearing (AMB) system.
Active magnetic bearing systems are used in rotor-dynamic machines for providing non-contact operation support of a rotating piece within a mechanical system. The non-contact feature of active magnetic bearings provides decreased rotational resistance on the rotor and reduced wear on the rotating system, leading to increased efficiency and rotating system component life.
At least some known active magnetic bearing systems include at least a pair of actuators, one or more position sensors, and a controller. Furthermore, some known active magnetic bearing systems are hybrid magnetic bearing systems that include active magnetic actuators and permanent magnets. The position sensors detect a radial position of the rotor, or an actual air gap distance, relative to at least one of the actuators. The air gap distance is communicated as a signal to the controller, which compares the actual air gap distance to a preferred air gap distance for operation of the rotor. The controller then emits an excitation current relating to a change in bearing current necessary to return the rotor to the preferred air gap distance.
As the rotor is spinning, vibrations are induced into the shaft from a number of sources, including, but not limited to, rotational unbalances in the shaft and external forces, such as fluid pressure differences acting on components coupled to the rotor. As the rotational speed changes, so does the frequency of the vibrations. As the frequency changes, the rotor experiences one or more bending modes. In general, the first bending mode shape includes two nodes and one anti-node, and the second bending mode shape includes three nodes and two anti-nodes. A node is a point on the rotor that does not oscillate for a particular mode shape. An anti-node is a point on the rotor between nodes where the rotor exhibits maximum displacement.
In some known active magnetic bearing systems, a node can fall between one of the actuators and the associated position sensor. The existence of the node between the sensor and actuator pair represents a 180° shift in the displacement detected by the sensor with respect to the displacement seen by the actuator. This displacement needs to be handled by the controller. Generally, theoretical models of the rotor are not accurate enough to detect each node location over a wide frequency range. Furthermore, at least some known tests methods of known rotors require dismantling the rotor-dynamic machine to isolate the rotor. In addition, some known methods of identifying bending modes rotors can only give results for a limited number of bending modes. Thus, identifying the number of, and the precise location of, vibration nodes for various bending modes of a rotor is very difficult.